1. Field of the Invention
The present invention relates to the manufacture of plastic bags or packages on a form, fill and seal (FFS) machine, particularly a vertical form, fill and seal (VFFS) machine, from a sheet of thermoplastic material, wherein each plastic bag or package includes a reclosable plastic zipper comprising a pair of mutually interlocking zipper profiles. More specifically, the present invention comprises a method and an apparatus for continuously and sequentially forming such bags or packages having such zippers disposed in a direction transverse to that of the filling tube of the FFS machine, and, consequently transverse to the direction in which the thermoplastic sheet progresses on the FFS machine during the production of the bags or packages. Further, the present invention comprises a reclosable plastic zipper which is particularly adapted to the practice of the invention.
2. Description of the Prior Art
The present invention relates to improvements in the package-making art and may be practiced in the manufacture of thermoplastic bags and packages of the kind that may be used for various consumer products, but which are particularly useful for food products which must be kept in moisture- and air-tight packages, free from leakage until initially opened for access to the product contents, which packages are then reclosable by zipper means to protect any remainder of the product therein.
The indicated art is fairly well-developed, but nevertheless remains susceptible to improvement contributing to increased efficiency and cost effectiveness.
One problem that still hampers the production of packages from continuous zipper-equipped sheet material is the difficulty in attaining a satisfactory sealing of the bag or package against leakage, where the zipper and area of film engaged by the zipper extends through the side (cross) seal areas separating one bag or package from the next. This problem occurs where the zipper is longitudinal with respect to the filling tube on the FFS machine, in which case the transverse, or side, sealing bars must flatten and seal the zipper at the same time as they are sealing the thermoplastic sheet material from which the packages are being made. The difficulty with which this is consistently and successfully achieved is reflected by the high occurrence of leaking packages.
Numerous attempts have been made to solve this problem. Among the approaches that have been taken is the substitution of a transverse zipper for the longitudinal zipper. Where such a zipper is provided, the transverse sealing bars associated with the FFS machine do not flatten the zipper as they are making the side seal, although they may seal the zipper to the thermoplastic sheet material without flattening it.
Several prior art patents are illustrative of such attempts. U.S. Pat. No. 4,878,987 to Van Erden shows a method and an apparatus for forming film capable of being converted into bags in a form, fill and seal machine or for providing bags attached to each other in end-to-end relationship including relatively advancing a continuous sheet of thin plastic film along a path having a fastener station therealong, moving first and second thermoplastic flexible fastener members laterally from opposite sides of the sheet to extend transversely substantially to the center thereof and attaching said fastener members to the surface of the sheet whereby the sheet can thereafter be formed into bags in a form, fill and seal machine or folded longitudinally to join the fastener members and to form a side seal to form a completed series of bags by cross-seaming the material at predetermined spaced intervals.
U.S. Pat. No. 4,909,017 to McMahon et al. shows a method of making a form-fill bag having a reclosable fastener thereon and a mechanism therefor wherein a continuous length of film is advanced and joined first and second fastener profile strips are laid laterally onto the film of a length substantially equal to one-half of the film width, the film is advanced and formed into a tube with the side edges folded together and seamed, the first profile strip is attached to the surface of the film prior to forming it into the tube and the second opposed interlocked profile strip is attached to the inner surface of the film after it is formed into a tube, and a cross-seam is formed in the tube above the closure strip to form the bottom of the succeeding bag and a completed bag is cut from the film by cutting below the bottom seam and above the fastener strips.
In both of these prior-art patents, the zipper profile or profiles is applied to thermoplastic sheet material before the material is introduced onto the form, fill and seal machine. As a consequence, the practice of these disclosed inventions has been hampered by an increase in sheet-handling problems brought about by the periodic disposition of the zipper profile or profiles thereon. To wit, the zipper profile or profiles has made it more difficult to smoothly wrap the sheet material about the filling tube of the FFS machine during the production of bags or packages. As such, these two prior-art inventions have not provided an acceptable solution to the side seal problem.
Another approach is shown in U.S. Pat. No. 5,111,643 to Hobock. This patent shows a fastener strip, with interlocking profiles of resilient material for purposes of enabling one to reclose a plastic bag after its initial opening, is secured to the bag as the bag is being formed over a cylindrical forming tube. To accomplish this, a continuous carrier strip to which the fastener strip material is mounted is passed into the interior of the forming tube to a port in the tube wall, the strip thereby passing to the outer surface of the tube where it travels part way around the tube circumference, reentering the tube interior through a second port. Between the ports, the carrier strip exposes the fastener strip mounted to it to the web from which the bag is being formed, and the fastener strip is heat fused to the web. A specially constructed combination strip which combines the carrier strip and the fastener strip material allows the two to readily separate once the fastener strip material is bonded to the web. This arrangement permits the fastener strip material to be applied to the bag web after the two interlocking halves of the fastener strip material have been joined.
It will be readily apparent that this procedure is quite awkward and complex. Unfortunately, it, too, has not provided an acceptable and workable solution to the side seal problem.
On the other hand, the present invention, to be described hereinbelow, affords a practical method and apparatus for manufacturing plastic bags and packages with transverse zipper on a FFS machine.